This invention relates to tone data compressing and expanding systems, which provide digital tone data and convert the digital tone data thus produced into an analog tone signal after superimposing the data on envelope data.
Recently, various digital electronic musical instruments, which convert digitally formed tone data into an analog signal through a digital-to-analog converter (hereinafter referred to as D/A converter), have been developed and put to practical use.
In such digital electronic musical instruments, however, the volume varies with the number of operated keys when producing a chord. For example, the volume ratio between the case of producing a single tone and the case of producing a chord consisting of eight different tones is 1:8 at the most. To be able to express digitally eight times the volume, three extra bits are required for a chord consisting of 8 tones compared to the case of a single tone. If a 12-bit D/A converter, for instance, is used and set to the 8-tone chord, the single tone is expressed by the lower 9 bits, i.e., the upper 3 bits are not used to express the single tone, so that the tone quality is greatly deteriorated.
Conversely, if the aforementioned D/A converter is set to the single tone output, i.e., if 12 bits are set to express a single tone, an overflow always results with a chord. To avoid this, it is necessary to provide D/A converters for the individual tones or use a 15-bit D/A converter than can express 8-tone chords as well.
The former case, i.e., using D/A converters for the individual tones leads to an increased cost. In addition, the electronic musical instrument system size is increased, which is undesired particularly for a portable electronic musical instrument. The latter case, i.e., use of the 15-bit D/A converter, is undesired from the standpoint that with the usual electronic musical instrument the expression of tone signal with 12 bits (corresponding to 72 dB of dynamic range) is sufficient.